“But it is best for Bird People,” replied the Doctor, “that the mother bird, who has to keep house and tend to the little ones, should not be too conspicuous.  She is best protected from enemies when her colors are plain, and especially when they match the foliage in which she sits on her nest.  If her mate has only himself to look out for, it does not so much matter how bright his plumage may be.  The colors of some birds are so exactly like their surroundings, that you might look long before you could find the sober, quiet female, whose mate is flashing his gay plumage and singing his finest song, perhaps for the very purpose of attracting your attention away from his home.  ‘Protective coloration,’ is what the Wise Men call it.”

“What makes all the different colors of birds, Doctor?” asked Rap.

“That is a hard question to answer.  It is natural for birds to have particular colors, just as some people have black eyes and hair, while others have blue eyes and yellow hair.  But I can tell you one thing about that.  Look at this Sparrow.  All the colors it shows are in the feathers, whose various markings are due to certain substances called ‘pigments,’ which filter into the feathers, and there set in various patterns.  The feathers are painted inside by Nature, and the colors show through.  You see none of these colors are shiny like polished metal.  But I could show you some birds whose plumage glitters with all the hues of the rainbow.  That glittering is called ‘iridescence.’  It does not depend upon any pigment in the substance of the feathers, but upon the way the light strikes them.  It is the same with the beautiful tints we see on a soap-bubble.  The film of water itself is colorless, but it becomes iridescent.  You might divide all the colors of birds into two classes—­those that depend upon pigments in the feathers, and those that depend upon the play of light on the feathers.”

“That’s pretty hard to remember,” said Nat; “but I know how a soap-bubble looks, though I never saw any birds look that way.  Please show us one.”

“I will show you two,” answered the Doctor, who then went to his glass case, and took out a Wild Pigeon and a Hummingbird.  “Look at the shining tints on the neck of this Pigeon, and see how the throat of this Hummingbird glitters when I turn it to the light.”

“That’s the prettiest color I ever saw,” said Nat, “and I can remember about it now.  But,” he added, thinking of the way he had seen hens mope when they were moulting, “does it hurt birds to lose their feathers, uncle?”

“It is probably not as comfortable as being nicely dressed, and sometimes they seem quite miserable, especially if they shed old feathers faster than new ones can grow to replace the lost ones.  Some birds, like Ducks, lose their wing-feathers all at once, and cannot fly for quite a while.  But Heart of Nature is kind to his children, as a rule.  Most birds shed their rowing feathers one at a time in each wing, so that they never lose their power of flight.  Now this will do for wings, tails, and feathers.  Come! what is the next thing you notice about this Sparrow?  Is it entirely covered with feathers?”